1. Field of the Invention
This invention generally relates to elastic boundary wave devices, resonators, and filters, and more particularly, to an elastic boundary wave device, resonator, and filter, in which Stoneley wave can be suppressed.
2. Description of the Related Art
There are known surface acoustic wave (hereinafter, referred to as SAW) devices, as one of the devices that utilize elastic waves. The SAW devices are for use in various circuits such as, for example, transmit bandpass filter, receive bandpass filter, local filter, antenna duplexer, intermediate frequency (IF) filter, frequency modulation (FM) modulator, and the like. The circuits process wireless signals ranging from 45 MHz to 2 GHz of frequency bands typically used by, for example, mobile telephones.
In order to improve the temperature characteristics, Japanese Patent Application Publication No. 2003-209458 (hereinafter, referred to as Document 1) discloses an elastic surface wave device, in which there is deposited on a piezoelectric substrate, a silicon oxide film having a different code in the temperature characteristic from the piezoelectric substrate. In an elastic surface wave device, elastic waves concentrate and propagate along the substrate surface. A foreign material adhered to the substrate surface will change or degrade the characteristics thereof, such as the change in frequency, increase in loss, or the like. For this reason, the elastic surface wave device is generally mounted on the hermetically sealed package. This makes it difficult to reduce the size of the device, leading to increased production cost.
Masatsune Yamaguchi, Takashi Yamashita, Ken-ya Hashimoto, Tatsuya Omori, “Highly Piezoelectric Boundary Waves in Si/SiO2/LiNbO3 Structure” Proceeding of 1998 IEEE International Frequency Control Symposium (U.S.) IEEE, 1998, p.p. 484-488 (hereinafter, referred to as Document 2) discloses a device that utilizes boundary waves propagating along the boundary between different mediums, instead of the surface waves, in order to improve the temperature characteristic and to reduce the size and production cost of the device. Document 2 discloses the boundary waves in the structure where a silicon oxide film and a silicon film are deposited on a 0-degree rotation Y-plate LiNbO3 (LN substrate), on the basis of the calculation results.
It is to be noted that Document 2 does not describe a method for producing an elastic boundary wave device having excellent characteristics suitable for practical use. As a problem against the practical use, there is an elastic wave excited as an unnecessary response, other than the elastic boundary waves utilized in the elastic boundary wave device. Such unnecessary response degrades the excellent frequency characteristics of the elastic boundary waves serving as a function of the elastic boundary wave device. As an example, FIG. 1 shows passband characteristics of a one-port resonator with an elastic boundary wave device. SH of FIG. 1 represents a response of a wave having a main component of Shear Horizontal (SH) wave utilized in the elastic boundary wave device. In other words, SH wave is used to fulfill the function thereof in the elastic boundary waved device. SV of FIG. 1 is observed as a response at a lower frequency side of SH. This is called Stoneley wave having a main component of Shear Vertical (SV) wave. As shown in FIG. 1, the Stoneley wave exciting as an elastic boundary wave is present at the closest frequency of the boundary wave having a main component of SH wave utilized in the elastic boundary wave device.